This invention relates to a printer mechanism and, more particularly, to an improvement over the devices described in U.S. Pat. Nos. 3,651,914 to Locke and 2,843,243 to Masterson.
The printer mechanism shown in the above patents comprises a rotating print member which in the preferred embodiment is a print wheel. The print wheel has a series of type characters arranged to lie in a helical path about the periphery of the wheel. The axis of rotation of the wheel parallels a print line and the wheel is continuously rotated about its axis as it is, at the same time, moved along its axis. The pitch of the helical path defined by the type characters is set to so compensate for the movement of the wheel along its axis that all the characters will scan past a first columnar print position during one revolution of the wheel and then scan past the second columnar print position during the second revolution and so on.
The advantages and flexibility of such a printer are extended by making it asynchronous in operation so that the wheel is selectively moved along its axis only after a character has been received from a data source and is ready to be printed. With this type of operation a printer which has a certain maximum printing rate can be made to print at any slower rate without the need for elaborate synchronizing circuits. Thus such a printer mechanism can be operated directly from a data line terminal, a keyboard or any of a number of other data sources with a minimum amount of interfacing problems.
It is, important to note that the transaction output rate of this type, as well as other types, of printing machines depends to a great extent on the tabulating and carriage return speed of the printer. The tabulating speed is the time it takes a printer carriage (or print wheel) to move from one area of a horizontal format to another area without printing.
In this regard, in the prior art, the mechanism to move a printing carriage to the right is provided by a lead screw, which is restricted to the speed of printing, and the mechanism to move the printing carriage to the left is provided by a spring, which is difficult to selectively initiate and terminate at desired carriage positions. Thus, it is an object of this invention to provide an additional motion source, independent of a lead screw, for producing motion of a printing carriage during tabulation and carriage return, which is fast and reliable in operation, flexible in application and is compatible with lead-screw motivated mechanisms.